Various information recording techniques have been developed with increase in volume of information processing in recent years. Particularly, the areal recording density of HDDs (hard disk drives) using the magnetic recording technique has been increasing at an annual rate of about 100%.
In order to achieve the high recording density in a magnetic disk for use in a HDD or the like, it is necessary to reduce the size of magnetic crystal grains forming a magnetic recording layer serving to record information signals, and further, to reduce the thickness of the layer. However, in the case of conventionally commercialized magnetic disks of the in-plane magnetic recording type (also called the longitudinal magnetic recording type or the horizontal magnetic recording type), as a result of the reduction in size of magnetic crystal grains, there has arisen a so-called thermal fluctuation phenomenon where the thermal stability of recorded signals is degraded due to superparamagnetism so that the recorded signals are lost. This has thus become an impeding factor for the increase in recording density of the magnetic disks. In order to solve this impeding factor, magnetic disks of the perpendicular magnetic recording type have been proposed in recent years.
In the case of the perpendicular magnetic recording type, differing from the case of the in-plane magnetic recording type, the easy magnetization axis of a magnetic recording layer is adjusted so as to be oriented in a direction perpendicular to a surface of a substrate. As compared with the in-plane magnetic recording type, the perpendicular magnetic recording type can suppress the thermal fluctuation phenomenon and thus is suitable for increasing the recording density.
When the magnetic recording layer has a hcp structure (hexagonal closest packed structure) in the perpendicular magnetic recording type, the easy magnetization axis is in a c-axis direction so that it is necessary to orient the c-axis in the normal direction of the substrate. In order to improve the orientation of the c-axis, it is effective to provide a nonmagnetic underlayer with a hcp structure under the magnetic recording layer as shown in Patent Document 1 (JP-A-2003-77122).
Further, in the perpendicular magnetic recording type, it is possible to improve the S/N ratio (Signal/Noise Ratio) and the coercive force Hc by forming the magnetic recording layer into a granular structure in which a nonmagnetic substance (mainly an oxide) is segregated between magnetic grains to form grain boundary portions, thereby isolating and miniaturizing the magnetic grains. Patent Document 2 (JP-A-2003-217107) describes a structure in which magnetic grains are epitaxially grown to form a columnar granular structure.
In the perpendicular magnetic recording type, an underlayer called an orientation control layer is provided for improving the crystal orientation of the magnetic recording layer. Ti, V, Zr, Hf, or the like is known as the orientation control layer, but as shown in Patent Document 3 (JP-A-H07-334832), it is known that particularly Ru (ruthenium) can effectively improve the crystal orientation of the magnetic recording layer to enhance the coercive force Hc. In the present where the increase in recording density is proceeding, Ru that can easily enhance the coercive force is predominantly used as the underlayer.
Further, it is known that even if a material is the same, the function of a film changes depending on an atmospheric gas pressure in a film forming process. Patent Document 4 (JP-A-2002-197630) proposes a structure having, as an undercoat film of a perpendicular magnetic layer, a layer containing ruthenium and formed in a high-pressure argon atmosphere and a layer containing ruthenium and formed in a low-pressure argon atmosphere. In Patent Document 4, it is described that the layer containing ruthenium and formed in the low-pressure argon atmosphere (around 1 Pa) exhibits an effect for higher orientation of the magnetic layer and the layer containing ruthenium and formed in the high-pressure argon atmosphere (about 6 Pa to 10 Pa) exhibits an effect for finer grains of the magnetic layer.    Patent Document 1: JP-A-2003-77122    Patent Document 2: JP-A-2003-217107    Patent Document 3: JP-A-H07-334832    Patent Document 4: JP-A-2002-197630